Regarding the effect of the Sb concentration on tensile strain level we found that tensile strain value slightly increases from 0.10% to 0.20% when the Sb source temperature varies from [r]
OPTICAL PROPERTIES OF SB DOPED GE FILM ON SILICON SUBSTRATE BY MOLECULAR BEAM EPITAXY TECHNIQUE Luong Thi Kim Phuong Hong Duc University, 565 Quang Trung street- Dong Ve district- Thanh Hoa city Email: info@123doc.org Abstract: To enhance the photoluminescence effeciency of the Ge film, we can apply a tensile strain or n doping in the Ge epilayers for modifying it’s energy band gap structure In this work, we combine both electron doping method from Sb source and inducing a tensile strain in Ge films Sb doped Ge grown on Si(100) substrate by molecular beam epitaxy technique The dependence of photoluminescence intensity on the substrate temperature in the range of 130-240oC and on the Sb source temperature from 240 to 300oC are investigated The active electron concentration obtained up to 2.5x1019cm-3 The tensile strain level in the Sb-doped Ge epilayers is twice larger than that of the P-doped Ge films using GaP solid source or PH3 gas precursor These results are significant in the realization of the Si based photoelectronic devices which are compatible with mainstream CMOS technology Keywords: n-doped Ge; Sb source; photoluminescence; tensile strain; optoelectronic I Introduction Photonic and optoelectronics play an important role in many fields of communication and information technology In the last years, researches on tensile strain Ge/Si with high electron doping have been developed [1-5] Although an indirect band gap material, it is demonstrated that the radiative recombination of Ge film could be greatly enhanced by inducing a tensile strain as well using n-doping method in Ge epilayers Addition, Ge is a semiconductor material with a high mobility of charge carriers Compared to Si the electron mobility of Ge is by a factor 2.5 higher, but it is more important that the hole mobility increases by a factor of 4 in Ge [6] Thus germanium-silicon based optoelectronic integrated circuit with microelectronics will open new opportunities on chip optical interconnect for high clock frequencies or cost effective solution for fiber to the home For this type of application, a necessary requirement is getting a high activated dopant concentration in Ge film With doping process, we can use dopant atoms from group V elements such as P, As or Sb Recent studies showed that P-doped Ge from specific GaP solid source could enhance the doping level up to 2x1019cm-3 [7-8] due to GaP cell produces P2 molecule with sticking coefficient 10 times higher than that of P4 from PH3 precursor gas [9- 10] Nevertheless, with this approach, tensile strain induced in Ge is negligible because the growth temperature is set up at low temperature and a rapid thermal annealing is applied after the deposition process Meanwhile thermal parameter is the key factor to induce tensile strain in the case of Ge on Si We employ the difference of thermal expansion coefficient between Ge and Si to accumulate a tensile strain in Ge film when growing the Ge epilayers on Si substrate at high temperature before cooling down to room temperature [11-14] Interestingly, the atomic radius of Sb is 16 % larger than that of Ge and when Sb atom substitutes Ge atom in the matrix, it may induce a tensile strain in Ge layers In this work, we investigated the Sb doping process in the Ge thin film which is grown by molecular beam epitaxy technique that allows incorporating of dopants above the solid solubility limit and at low temperature to restrict the diffusion effect We also study the accumulation of the tensile strain in Ge epilayers due to the incorporation of Sb atoms II Experimental set up Ge epilayer growth was implemented in a standard MBE system with a base pressure lower than 2.10-10 torr The growth chamber is equipped with a 30keV reflection high-energy electron diffraction (RHEED) apparatus allowing to observe in-situ and in real-time the Ge growth mode Ge was evaporated from a two zone heated Knudsen effusion cell with deposited rate in range of about 2-5nm.min-1 The substrate surface was flat, n-type Si (001) wafers Cleaning of the substrate surface follows chemical step and thermal treatment step [15] After these steps, the Si surface exposes a well-developed (2x1) reconstruction The substrate temperature was estimated using a thermal-couple in contact with the backside of the Si wafer with accuracy of about 20oC After epitaxial growth, all sample were annealed to activate the dopant atoms for occupying the substitution sites in Ge lattice as well ameliorating the crystalline quality of Ge film The growth of pure Ge on Si substrate is technologically challenging because of the lattice mismatch of 4% between Si and Ge Two steps growth method is applied consisting of an 50nm thick of smooth Ge buffer layer grown at low temperature (at 270oC) follow by a thick Sb doped Ge layer at the growth temperature in the range of 130-240oC [11] The strain level in the Ge epilayers was deduced from X-ray diffraction (XRD) measurements performed using a diffractometer (Philips X’pert MPD) equipped with a copper target for Cu-Ka1 radiation (=1.54059A°) The angular resolution is 0.01o The PL is measured with a 532nm laser focused on the sample surface The PL signal is measured with an InGaAs detector PL spectra were recorded at room temperature The Active antimony concentration is calculated by mean of using both Hall effect measurement and band gap narrowing phenomenon III Results and discussion The growth chamber is equipped with RHEED, which is powerful technique allowing us to monitor in real time the change of both the surface morphology and the surface structure Figure 1 Representative RHEED patterns taken along the [100] azimuth of Sb doped Ge film grown on the Si(100) substrate Sb source temperature varies in the range of 240-280o These are keys parameter affecting on photoluminescence ability of Ge film We first investigate the surface morphology of sample which is grown at the substrate temperature of about 160oC and Sb source temperature varies from 240 to 320oC Figure 1 displays representative RHEED patterns observed when the Sb cell temperature was set up from 240 to 280°C the RHEED pattern becomes slightly 3D but half-ordered ½ streaks characteristic of the 2x1 reconstruction of the Ge surface are still present This means that the growth of the corresponding Sb doped Ge film is still proceeded via a layer-by-layer mode and sample surface is smooth and uniform The present of 3D seed resulting from the Sb incorporation which has the atomic radius lager than that of Ge matrix Figure 2 Dependence of the room-temperature photoluminescence spectrum on the Sb source temperature We now study the influence of the Sb source temperature on the photoluminescence intensity Figure 2 displays the evolution of the room-temperature photoluminescence spectrum versus the temperature of the Sb cell For all samples, the substrate temperature is chosen to be 160°C and the film thickness is 600 nm The temperature of the Sb source increases from 240 to 300 °C After growth, all samples were annealed in the growth chamber at 600°C during 30 seconds to activate dopants and reduce the dislocation density As can be seen from the figure, the photoluminescence intensity increases with increasing the temperature of the Sb source from 240 to 280°C and the highest PL intensity is obtained at 280°C For the Sb source temperature at 300°C, the PL is found to dramatically decrease To understand the above evolution, we found that when the Sb source temperature increases, the amount of Sb atom incorporating into Ge lattice increases Thus the activated electron concentration resulting the increase of PL intensity However, if the Sb atoms doped in the Ge film exceed the critical value, the crystal structure of Ge matrix will be deteriorated and forming the Si rich clusters This result correlates with the RHEED observation of the Sb doped Ge sample when Sb source temperature above 300oC The streaky patterns disappear and crystal structure turns into amorphous state with the presentative rings (not show here) One of the most important growth parameter is the substrate temperature To investigate the effect of the doping level versus the substrate temperature as well the role of sticking coefficient of Sb on Si substrate, we have therefore fixed the Sb source at a constant Figure 3 Evolution of the room-temperature photoluminescence spectrum versus the growth temperature All the samples have the same a film thickness of 600nm temperature of 280 °C Figure 3 displays the evolution of the PL intensity versus the substrate temperature As can be seen, the PL intensity is found to increase with decreasing the substrate temperature from 240 to 130 °C and the highest intensity is obtained at 160 °C At growth temperature higher than 190oC, the PL intensity decrease dramatically because of the large segregation of Sb in Ge film [16] The previous studies show that, when Ge is under degenerate doping, i.e when the n- type doping concentration is higher than 1x1019 atoms.cm-3, a clear red shift in emission wavelength is observed The phenomenon is called ‘band gap narrowing’ [17-19] Thus, one Figure 4 Evolution of Ge peak corresponds to the maximum wavelength of the direct transition emission versus the growth condition a) on the Sb source temperature; b) on the substrate temperature can evaluate the activated electron concentration from the shift of the emitted wavelength Figure 4a shows the evolution of Ge peak wavelength (corresponding to the maximum photoluminescence intensity) versus the Sb source temperature As can be seen, when the Sb source temperature increases from 240 to 280oC, the Ge peak increases from 1620 to 1624nm Continuing to inrease the temperature to 300oC, the peak wavelength decreases due to the crystal quality of Ge film Figure 4b presents the dependence of Ge peak wavelength on the substrate temperature From the figure we can observe that the peak varies from 1617 to 1631nm with the increase of substrate temperature from 130 to 160oC For the further increase of growth temperature, the Ge peak decrease because of the large segregation of Sb atoms in Ge film at high temperature At 160 °C of substrate temperature and 280oC of Sb Figure 6 X-ray diffraction spectra of Sb doped Ge epilayers growth on the Si (100) substrate The film thickness is 600nm Figure 5 Dependence of carrier’s concentration on the measurement temperature of Sb doped Ge epilayers on the SOI substrate cell, the PL spectrum peak is located at around 1631 nm (i.e the corresponding energy is 0.761 eV) arising from the direct band gap emission narrowing at high n-doping levels This transition can be attributed to arise from the direct band gap radiative recombination of the n- doped Ge layer As compared to the energy maximum around 0.810 eV of unstrained and un- doped Ge, we observe here a redshift of 49 meV, which can be attributed to band gap (silicon on oxide) substrate Taken into account a tensile strain of about 0.20 % in our samples (deduced from XRD measurements that will be discuss in the next part) and with a maximum of the PL spectrum located at 1631 nm, we can deduce an activated electron concentration of about 2.5x1019.cm-3 The value of the electron concentration is in good agreement with that obtained from Hall measurements shown in figure 5 We note that for Hall measurements, we have grown thick samples (1150 nm) on a SOI substrate (Silicon On Insulator) to avoid any transport contribution coming from the substrate As we mention above, one of the reason to choose Sb as a dopant element in Ge is its atomic radius Due to the atomic radius of Sb is 16 % larger than that of Ge, a compensation of the local strain fields is expected and when the Sb concentration is high enough, the Ge layer can be compressively strained Figure 6 show the evolution of the strain state in the Ge layer prior to annealing and after annealing at 600°C for 30 sec It’s worth noting that, before annealing (the blue curve) the Ge layer is compressively strained by a value of about -0.20 % After annealing, the strain in the Ge layer becomes tensile of about 0.20 % (the pink curve) The XRD measurement also reveals that the film quality has greatly improved after thermal annealing, since the intensity of the (004) reflection has increased and its half-width decreases Regarding the effect of the Sb concentration on tensile strain level we found that tensile strain value slightly increases from 0.10% to 0.20% when the Sb source temperature varies from 240 to 280oC (the red curve and the pink curve respectively) Of particular interest, in addition to the increase of the total electron concentration, Sb also allows to enhance the final value of the tensile strain in the Ge film IV Conclusion We have effectuated Sb-doping process in Ge film by molecular beam epitaxy system The growth conditions are studied in which the substrate temperature varies from 130 to 240oC and Sb source temperature is in the range of 240-330oC The optimal growth condition for the highest PL intensity obtaining at the substrate temperature is about 160oC and Sb cell temperature is 280oC respectively Owning to MBE technique, we can manufacture the Sb-doped Ge film at low temperature to avoid the large segregation of Sb in Ge The activated dopant concentration achieved up to 2.5x1019cm-3 after rapid thermal annealing at 600oC in 30s The effect of Sb element cooperates in Ge epilayers on tensile strain value is also investigated Due to the atomic radius of Sb is 16 % larger than that of Ge, Sb induced a tensile strain level of about 0.20% in Ge film Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2015.106 We also thank Prof V Le Thanh, Prof Philippe Boucaud, Dr A Ghrib and their group for supporting this work References [1] M Oehme, J Werner, E Kasper, M Jutzi and M Berroth, 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in Ge LED’s On Si Substrates”, Opt Exp 21 (2013) 2206 ... diffraction spectra of Sb doped Ge epilayers growth on the Si (100) substrate The film thickness is 600nm Figure Dependence of carrier’s concentration on the measurement temperature of Sb doped Ge. .. value of the tensile strain in the Ge film IV Conclusion We have effectuated Sb- doping process in Ge film by molecular beam epitaxy system The growth conditions are studied in which the substrate. .. (1150 nm) on a SOI substrate (Silicon On Insulator) to avoid any transport contribution coming from the substrate As we mention above, one of the reason to choose Sb as a dopant element in Ge is